Building a Garden Pond for Koi

The objectives of designing a koi pond

A koi pond is designed and built with displaying the koi in a clear and healthy environment being the main objective. Everything is planned with the koi in mind.

As koi are unlike any other pond fish, the design of a koi pond is quite unique.

1. Koi grow to be large fish.

Koi can grow to over 1 metre in length and require their accommodation to offer wide open swimming space. By providing as large a pond as possible (both in area and depth), then koi growth and development will not be impeded, allowing your koi to reach their full potential.

2. Koi are notoriously weak.

Koi are the product of many generations of selective line breeding, where closely related fish are crossed to exhibit the desirable features, such as pattern and colour. An unavoidable phenomenon associated with such a high degree of in-breeding is the reduction in the vigour of koi, particularly the more delicately patterned and high grade specimens. Essentially, koi are bred for their external appearance, and not their internal vigour.

Keeping koi is not as nature had intended, where the fittest survive. So when we keep koi we play a key role in making sure the weakest survive, and to do that we need all the help we can get – which starts with a well designed koi pond.

Because koi are weaker than other pond fish, they must be protected from stress, – the precursor to health and disease problems. As water quality is the most common cause of stress, the koi pond must be designed to provide your koi with the best water conditions. The larger a pond, the more stable an environment your koi will experience. Larger ponds hold more volume which will be less prone to rapid temperature changes and will also play a large diluting effect on the build up of pollutants. Koi will thrive in a pond where conditions are favourable and stable, they cannot tolerate rapid change and have a habit of letting you know if they do.

3. Maintaining ideal water conditions.

Koi keepers have a reputation of wanting to show off and compare their filter systems just as much as their koi, and rightly so, it is an essential part of the pond’s design. A filter system and a koi pond are inseparable and when designing the ideal environment for koi they should be considered together. A filter will maintain the clarity of pond water while removing and breaking down the toxic fish waste. It is the engine room of the pond, and the better it is designed the better it will perform, giving your koi superb water quality and your peace of mind.

4. An unplanted pond

The first impressions when viewing a koi pond are its clean lines, where the sides are vertical and shape is simple and uncluttered. A koi pond can be quite clinical in appearance, looking unnatural in its unplanted state. Plants and koi do not make good bedfellows and the decision to go plant-free should be made early on. Aquatic plants are potted or basketted up in soil which is topped off with a layer of gravel to retain the soil and weight the plant to the bottom. To koi, the aquatic pigs of the pond, a row of planted baskets presents the same temptation to pigs smelling out truffles. They’ve just got to root around – it’s their instinct, and they won’t stop until they have removed, tasted, and stirred up the aquatic soil, uprooting plants until they float to the surface. Not really a recipe for a tranquil, crystal clear pond. So plants are not an option. This will mean that green water may soon take hold (unless controlled with a UV), but if the pond is crystal clear, then blanket weed will thrive with its monopoly on sunlight and nutrients. This can be controlled with the strongest of algicides with no need for any consideration for other aquatic plant life.

5. Go for depth.

Besides being steep sided (there is no need for planted marginal shelves), koi ponds are the deepest of all garden ponds. Building a koi pond is back breaking work because of the extra spoil that is to be removed (and this is why many leave it to the professionals). An absolute minimum depth of 3 feet should be provided for koi, with many being 6 feet deep and over. A little ingenuity at the design stage can save some digging by raising the pond above the ground, (2 foot above the ground and 4 foot of digging!).

Considerations when going so deep will include:

1) A 6 foot deep pond will need an 8 foot deep hole to allow for foundations and pipework (bottom drain for filter)

2) Drains. When purging the filters that sit alongside the pond, ensure that there is sufficient fall from the bottom of the filter to the main sewer. This will allow the easy draining and cleaning of the filter chambers throughout the life of the pond.

Siting and setting up a typical garden pond.

Some of the most memorable ponds are those which look as though they have been planted by mother nature herself. Appearing as though generations of weathering and evolution have shaped an aquatic hollow, to nestle in true balance with its surroundings.

We too appear to be in tune with nature’s hand as, almost by instinct, we can soon pick out the man-made impostor from nature’s own creation. We seem to be programmed to detect what is natural and what is not and yet still find it difficult to mimic the subtleties of nature’s creative hand when forming our own garden pond. Nature is a perfectionist at positioning, landscaping and planting and to be able to splice a piece of the ‘real thing’ in our gardens, we too must address these same areas.


Unfortunately the task of positioning a pond is not as simple as asking the question ‘where would a pond occur within my plot of land if it were natural?’. Most natural ponds will form in hollows in the ground, where the land drains itself collecting to form a pool but most gardens are reasonably flat this is not an option unless substantial landscaping can be carried out. Furthermore, such low lying plots tend to be areas where a high water table exists, making it difficult to install a pond or liner due to incessant rising water filling the excavation.

Although areas where water is naturally abundant are often well populated with trees, a garden pond should be positioned so as to avoid leaf-fall wherever possible. Particular attention should be paid to the more notorious trees whose leaves, blossom or berries may prove toxic if falling into a pond. These include poplars, oak, elders, willow and yew trees. Furthermore, keeping clear of trees should make digging a pond much easier by avoiding their roots. If tree roots are encountered and removed during the excavation then the tree may eventually suffer or subsequent years of regenerative root growth installation could pose a risk of puncturing the pond at a later date.

Similar considerations should be made if planning a pond to be adjacent to other immovable obstructions such as walls and other boundaries. Avoiding these will again make digging much easier and will avoid disturbing foundations.

Near or Far?

The viewing point for a new pond will determine its ultimate location. Some prefer a pond installed closer to the house where it can be enjoyed form inside the house or conservatory. This makes getting an electricity supply to the pond relatively simple and the house will also offer a degree of shelter from the elements, particularly in winter. Issues to overcome when siting a pond close to a house include avoiding drains during construction and ensuring that the house does not shade the pond completely of the necessary sunlight.

A further consideration when deciding where to site a pond is what to do with the spoil that is removed from the hole. The spoil can quite simply be stacked behind the pond to form a raised area for a rockery or waterfall. It can be difficult to build a raised backdrop for a pond close to a house as the mound can soon look like a misplaced bump in the garden. It is easier to achieve a more natural feature with a raised rockery or waterfall when positioning a pond further away from the house. This provides a raised backdrop in front of which the whole garden can be presented.

In addition, a pond positioned further away from the house can be landscaped more easily to blend in with the rest of a garden and can form a separate aquatic oasis to a garden which must be visited specifically to be enjoyed.

Pond shape.

Taking an example from natural pools, you will notice that they are shaped very simply, with sweeping curves and edges, perhaps shelving gently to form a ‘beach’ area. A planted garden pond should aim to follow a similar style, avoiding nooks and crannies which will form dead spots of still water and also prove difficult when laying a pond liner, creating creases and folds and using up extra material. Dig your pond to suit the area of land available, keeping the perimeter smooth and gently sweeping.


The depth of a planted pond will vary across its area. Shelving around the edge should allow at least 9” of depth below the surface to allow space for planted baskets. The shelving need not be continuous around the pond’s perimeter and deeper shelving can be constructed for larger marginal plants.

The centre of the pond will be the deepest, aiming for a minimum depth of 2 feet. If the pond is not sufficiently deep then it may be liable to freeze in winter to such an extent that only a limited amount of water will remain below the ice in which the fish are to survive. Furthermore, the greater the volume of water that is contained in a pond, the more stable its temperature, preventing it from cooling down or heating up too rapidly. A larger volume of water will also enable you to keep more fish in a healthy pond environment. The solution to pollution is dilution.


All coldwater ornamental pond fish are compatible with each other, but some are not compatible with a planted pond. Koi and ghost carp are notorious for being destructive to plants. Their inquisitive feeding leads them to investigate any soft substrate, which unfortunately includes planted baskets. These can also grow rapidly and to a great size, making them less suited to a balanced planted pond. Any of the goldfish varieties (excluding the fancy goldfish) can be stocked. These include goldfish, comets, shubunkins and all variations in between! Golden Orfe and tench also make worthy pond companions.

The pond is best stocked on the basis of the pond’s area rather than its volume as this best determines the pond’s capacity for fish. As the balance of every pond is different, there are no fixed rules regarding the stocking levels for ponds. A pond will be pushed to its limit when it is hottest and will accommodate more fish when the pond is overwintering. To be safe, you should stock with the hottest conditions in mind and in doing so, aim for 3 inches of fish (excluding tail) per square foot of clear water surface as a rough guideline.

As with any pond, stock gradually, a few fish at a time and monitor water quality to make sure that your pond remains in balance.

Pond Equipment

The beauty of creating a planted pond is that very little pond equipment is required to achieve a stunning result.

1. Pond. The two most popular alternative materials of making a pond are pre-formed fibreglass ponds or flexible PVC liners. Although the preformed ponds are shaped and ready to go, they tend to be more expensive than liners and are quite restricted in their design. A liner however puts you in complete control and can be bought to fit any excavation you choose to make offering a lifetime guarantee of 25 years.

A liner is usually bought off-the-roll. The area required is calculated by measuring the greatest length and width and adding double the depth to the length and width giving the dimension of the liner required.

2. Pond Pump.

If the stocking of fish is likely to exceed what the pond can sustain naturally, then a water pump is required. If however you are only looking to stock a small number of fish in a planted pond then even a pump should not be necessary. As a pump will be working in partnership with the plants and the ecology of the pond, a small, internal foam filter should suffice, which is unobtrusive and easy to install. The pump can be fitted with a fountain head for effect if desired. If a larger filter is required, then an external box biofilter can be installed and used to feed a waterfall. The pump should be installed to circulate the pond’s volume once every 2 hours.

Kill blanketweed and string algae.